linux-kernel - Re: [PATCH v2] mm/memory: Don't require head page for do_set

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Date: Tue, 11 Jun 2024 11:06:22 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Bresticker <abrestic@...osinc.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd()

On Tue, 11 Jun 2024 17:33:17 +0200 David Hildenbrand <david@...hat.com> wrote:

> On 11.06.24 17:32, Andrew Bresticker wrote:
> > The requirement that the head page be passed to do_set_pmd() was added
> > in commit ef37b2ea08ac ("mm/memory: page_add_file_rmap() ->
> > folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
> > finish_fault() and filemap_map_pages() paths if the page to be inserted
> > is anything but the head page for an otherwise suitable vma and pmd-sized
> > page.
> > 
> > Fixes: ef37b2ea08ac ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
> > Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
> > ---
> >   mm/memory.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0f47a533014e..a1fce5ddacb3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4614,8 +4614,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> >   	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> >   		return ret;
> >   
> > -	if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
> > +	if (folio_order(folio) != HPAGE_PMD_ORDER)
> >   		return ret;
> > +	page = &folio->page;
> >   
> >   	/*
> >   	 * Just backoff if any subpage of a THP is corrupted otherwise
> 
> Acked-by: David Hildenbrand <david@...hat.com>

You know what I'm going to ask ;) I'm assuming that the runtime effects
are "small performance optimization" and that "should we backport the
fix" is "no".